Dynamics of a Near- Surface Pipeline Tow

Abstract

1. Introduction This project arose from a series of experiments carried out by the Melbourne node of the Australian Maritime Engineering Cooperative Research Centre (AMECRC) at Monash University in 1993 for the Australian Marine and Offshore Group Pty Ltd. During these experiments, a model pipeline string simulating a 400 m long full-scrde pipe was tested for a 15 km near-surface tow to deep water. The pipe was made approximately neutrally buoyant by the use of buoyancy cans and chains. It then acted as a beam excited by the waves. The main concerns for this operation were that, under all expected wave conditions, the pipe stresses should not be excessive, and an estimate for the power required to tow the pipe should be determined for a range of waves and towing speeds. It was considered that an experimental study was necessruy because of uncertainties and the non-linear nature of the processes. This paper reports on an experimental and computational sequel to the original testing project, aimed at developing and veri&ing a numerical model of the pipeline response to wave action. The overall aims of the research conducted were to uncover the relationships controlling the transverse vibration of a smooth pipeline at near-surface depths in waves by including towing speed and back tension. In the following sections, the problem is specified, the numerical method developed and sample output fi-om the numerical method presented. Sections 5 and 6 describe the experimental procedure and results. Sections 7 and 8 compare the numerical results, draw conclusions and suggest some applications of the results. 2 Predictions of Pipe Motions 2.1 Problem Formulation The first task in predicting the pipe motions was to derive the differential equation governing the motion of the pipe. This was done by balancing the forces on a small element of pipe. The element of pipe considered is shown in Figure 1. This is the standard element used in vibrational analysis of a beam with the inclusion of the hydrodynamic forces. In the figure, V is the internal shear force due to the adjacent pipe elements, M is the internal bending moment and T is the tension force supplied by the adjacent pipe elements. The pipe is considered to be neutrally buoyant. Hence, its weight has not been included. The virtual mass in the equation consists of the actual mass of the pipe element plus the added mass. The hydrodynamic loading for a slender cylinder is assumed to be described by the Morison equation with drag and inertia coefficients taken from Sarpkaya and de St.Q. Isaacson (1981). 2.2 Equations of Motion The final differential equation is (Available In Full Paper) where p is the water density, d is the pipe diameter and Cd and Cm are the Morison drag and inertia coefficients. Also in Equation (l), EI is the bending stiffness of the pipe, T is the tension in the pipe, A is the cross-sectional area of the pipe and & is the velocity of the water due to wave action.